This invention relates to casting of molten metal into solidified forms such as sheet, plate, bar, ingot or strips and more particularly, this invention relates to improved heating of nozzles or tips for supplying molten metal to casters such as wheel, roll, belt or block casters.
For purposes of supplying molten metal, e.g. aluminum, to a continuous caster, for example, a roll caster, a casting nozzle is used having a tip which extends into the casting rolls. Such tips are shown, for example, in U.S. Pat. Nos. 3,774,670; 4,526,223; 4,527,612; 4,550,766 and 4,798,315.
Casting nozzles have been fabricated from various refractory materials. For example, U.S. Pat. No. 4,485,835 discloses that the part of the nozzle coming in contact with the molten metal is a refractory material comprised of silica, asbestos, sodium silicate and lime, which material is available under the trade names Marinite and Marinet. Further, U.S. Pat. No. 4,485,835 discloses that while the refractory nozzles exhibit good thermal insulation and low heat capacity, it is not very homogeneous in terms of chemical composition and mechanical properties. In addition, it adsorbs moisture and is subject to embrittlement or low mechanical strength upon preheating to operating temperature which allows such nozzles to be used only once. Further, such materials frequently outgas and experience cracking upon heating, both of which are undesirable characteristics for successful caster nozzle performance.
Refractory materials used to fabricate the casting nozzles have not been satisfactory for other reasons. For example, often the refractory material is reactive or subject to erosion or dissolution by the molten metal, e.g., aluminum, being cast, and this results in particles of refractory or reaction products ending up in the cast product.
Another problem with refractory material is that it often cannot maintain the proper strength level under operating conditions. This can result in sag or change in its dimensions which adversely affects or changes the flow of molten metal to the casting mold. That is, the flow of molten metal across the tip of the nozzle does not remain uniform. This can change the freeze front and thus properties can change across the width of the product. Change of the internal dimensions of the nozzle can result in metal flow disturbances and surface defects on the resulting sheet or plate such as eddy currents, turbulence or otherwise non-uniform flow through the nozzle.
Yet, another problem with refractory-type nozzles is that often they are not reusable. That is, after molten metal has been passed once through the nozzle and the caster has been shut down, the nozzle is not reusable. Thus, a new nozzle, even if it has only been used for a short time cannot be used again. This greatly adds to the expense of operating the caster.
Reproducibility with respect to the dimensions of the refractory nozzles is a problem. For example, some nozzles may be found to work acceptably and others have been found to be unacceptable because tolerances are difficult to maintain. This leads to a very high rejection rate for nozzles which again adds greatly to the cost of operating the caster.
Before molten metal is poured into the nozzle, it is preferred to heat the nozzle to minimize warpage and to avoid prematurely cooling the molten metal. However, with refractory materials, it is difficult to heat the nozzle uniformly. Traditionally, nozzles have been heated by impinging a gas flame on the nozzle or placing an electric heating unit inside the channels of the nozzle. However, this results in heating non-uniformly within the same nozzle and further results in heating non-uniformly from nozzle to nozzle. Further, this form of heating has the problem of open flame or the extra step of removing the heating unit prior to pouring molten metal thereinto. Additionally, these methods of preheating the nozzle do not readily permit the use of such heating means after the metal has been introduced or control of molten metal temperature after it enters the nozzle.
To minimize sagging experienced with nozzles, the above-noted U.S. Pat. Nos. 4,526,223; 4,527,612; 4,550,766 and 4,550,767 disclose the use of spacers. U.S. Pat. No. 4,153,101 discloses a nozzle having a lower plate and an upper plate separated by cross pieces. Outside of the nozzle is an extension on either side of the nozzle referred to as a cheek which is divergent. U.S. Pat. No. 3,799,410 discloses the use of baffles to control the flow of molten metal to a casting machine. U.S. Pat. No. 5,164,097 discloses the use of a solid titanium liner in a crucible and nozzle for casting molten titanium.
Traditionally metals have not been used for nozzles or containers and the like because molten metal such as molten aluminum can dissolve the metal. In addition, most metals do not have the desirable combination of low thermal conductivity and low thermal expansion coefficients necessary for use in certain applications with molten metal. Refractory materials have not been used because they are subject to thermal shock, have low strength, are brittle and have low toughness, all of which are necessary for applications such as nozzles.
Another common problem experienced in the casting of molten aluminum is the formation of intermetallic precipitates. For example, aluminum carbide can form on the nozzle substrate material. Thus, it is desirable to utilize a substrate material that does not promote precipitation of intermetallic compounds and to use a nozzle design that discourages plugging due to precipitation of such compounds.
From the above, it will be seen that there is a great need for a nozzle which solves these problems and permits continued use or permits cleaning for continued use. The present invention provides such a nozzle which can be fabricated for use with any type of caster, including wheel, roll, block or belt casters. Further, the nozzle, in accordance with the invention has the advantage that it can be uniformly electrically heated by induction or resistance heating, for example, to bring the nozzle to operating temperature thereby avoiding thermal shock when molten metal is introduced and avoiding the problems attendant thereto.